Multi-modal fluorescence imaging flow cytometry system

ABSTRACT

In one aspect, the present teachings provide a system for performing cytometry that can be operated in three operational modes. In one operational mode, a fluorescence image of a sample is obtained by exciting one or more fluorophore(s) present in the sample by an excitation beam formed as a superposition of a top-hat-shaped beam with a plurality of beams that are radiofrequency shifted relative to one another. In another operational mode, a sample can be illuminated successively over a time interval by a laser beam at a plurality of excitation frequencies in a scanning fashion. The fluorescence emission from the sample can be detected and analyzed, e.g., to generate a fluorescence image of the sample. In yet another operational mode, the system can be operated to illuminate a plurality of locations of a sample concurrently by a single excitation frequency, which can be generated, e.g., by shifting the central frequency of a laser beam by a radiofrequency. For example, a horizontal extent of the sample can be illuminated by a laser beam at a single excitation frequency. The detected fluorescence radiation can be used to analyze the fluorescence content of the sample, e.g., a cell/particle.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to Provisional Patent Application No.62/240,894, filed Oct. 13, 2016, the contents of which are incorporatedherein by reference in their entireties.

BACKGROUND

The present invention relates generally to devices and methods forfluorescence analysis of samples, and more particularly to devices andmethods for fluorescence-based flow cytometry.

Fluorescence imaging has a variety of biomedical applications, forexample, in obtaining information regarding molecular composition ofbiological specimens. In biomedical flow cytometry, fluorescenceradiation emitted by exogenous and/or endogenous cellular fluorophoresis collected and analyzed to derive information about chemical and/orphysical properties of cells.

In conventional fluorescence-based flow cytometry, however, theacquisition of blur-free images of cells flowing at a high speed, orother fast phenomena such as sub-millisecond biochemical dynamics, canbe challenging. In particular, the weak optical emission of manyfluorophores coupled with the short exposure of the sample at imagingframe rates in the kilohertz range renders the acquisition of blur-freeimages difficult. Moreover, many conventional systems operate in onlyone imaging mode and hence fail to provide sufficient flexibility forsample analysis.

Accordingly, there is a need for enhanced methods and systems forfluorescence analysis, and in particular a need for enhanced methods andsystems for performing fluorescence-based flow cytometry.

SUMMARY

In one aspect, a system for performing cytometry is disclosed, whichcomprises a laser for generating a laser beam having a central frequencysuitable for exciting at least one fluorophore, an acousto-opticdeflector for receiving the laser beam and generating a plurality ofangularly separated laser beams each having a radio frequency (RF) shiftrelative to said central frequency. The angularly separated laser beamsinclude a local oscillator beam (LO beam) and a plurality of RF combbeams, where each of the beams exhibits a radiofrequency shift relativeto the central laser frequency. An optical element directs the LO beamalong a propagation path different than propagation paths of the RF combbeams. A top-hat beam shaper receives the LO beam and imparts thereto atop-hat intensity profile, e.g., along one direction in a planeperpendicular to its propagation direction. The system further includesa beam splitter that receives the top-hat-shaped LO beam and the RF combbeams and provides a combined beam by spatial overlap of said beams, andat least one optical element (e.g., a lens) for directing the combinedbeam onto a sample, which can comprise a plurality of cells at leastsome of which are associated with said fluorophore, such that LO beamconcurrently illuminates a plurality of spatial locations of the sample,e.g., as the sample flows through a flow cell, and each of said RF combbeams illuminates a different one of said spatial locations to elicitfluorescence radiation from said fluorophore, if present, at saidspatial locations. The fluorescence radiation emitted from each of saidsample locations exhibits a beat frequency corresponding to a frequencydifference between said LO beam and one of the RF comb beamsilluminating that sample location.

In some embodiments, the frequency differences between theradiofrequency (RF) shifts are less than a FWHM (full width at halfmaximum) of a spectral absorption peak of said fluorophore. By way ofexample, the radio frequency shifts are in a range of about 10 MHz toabout 250 MHz, e.g., in a range of about 50 MHz to about 150 MHz. Insome embodiments, the radiofrequency shifts are separated from oneanother by a frequency in a range of about 0.1 MHz to about 4 MHz

In some embodiments, the above system further comprises a lens disposeddownstream of the beam splitter for focusing the top-hat-shaped LO beamand the RF comb beams onto an intermediate plane such that the top-hatLO beam has overlap with each of the RF comb beams. The top-hat-shapedLO beam's intensity profile at each overlap location can besubstantially identical with its profile at another overlap location. Insome cases, the LO beam has a linear extent in this intermediate plane(e.g., an extent along a horizontal dimension in this plane) that issubstantially equal to a linear extent of the angularly separated RFcomb beams (e.g., the maximum horizontal distance between the RF combbeams). The top-hat-shaped LO beam has preferably a substantiallyuniform polarization in the top-hat direction (along the elongateddirection of the beam).

By way of example, in some embodiments, the RF comb beams have aGaussian intensity profile, and the top-hat-shaped LO beam has anintensity that is substantially equal to the maximum intensity of theGaussian intensity profile. Further, the polarizations of the RF combbeams and the top-hat shaped LO beam can be aligned.

The system can include a radio frequency generator for concurrentlygenerating radio frequencies corresponding to the radio frequency shiftsand applying the radio frequencies to the acousto-optic deflector togenerate said LO and RF comb beams. In some embodiments, the radiofrequency generator includes a direct digital synthesizer (DDS) RF combgenerator. In some embodiments, an electronic power amplifier canamplify the radiofrequency drive signals generated by the radiofrequencygenerator for application to the acousto-optic deflector. A controllercan control the radiofrequency generator, e.g., for operating the systemin different operational modes, as discussed further below, and/or foradjusting the amplitude and/or frequency of the drive signals applied tothe acousto-optic deflector.

The system can further include one or more photodetectors (e.g., one ormore photomultiplier tubes) for detecting the fluorescence radiation, ifany, emitted from the sample and generating a time-domain fluorescencesignal. In some cases, an appropriate filter (e.g., an optical bandpassfilter) is disposed in front of the photodetector to allow transmissionof a fluorescence frequency of interest while blocking unwantedradiation frequencies.

In some embodiments, the excitation radiation (i.e., the combination ofthe LO beam and the RF comb beams) can concurrently excite multiplefluorophores within the sample. In some such embodiments, the system caninclude multiple photodetectors, each of which is used to detectfluorescence radiation emitted from one of those fluorophores. In someembodiments, an appropriate filter, e.g., a bandpass filter, is disposedin front of each of the photodetectors that allows transmission ofradiation at the fluorescence frequency of interest to the respectivedetector while blocking unwanted radiation.

In some embodiments, an objective lens can receive the excitationradiation (i.e., the combination of the LO beam and the RF comb beams)via reflection by a dichroic mirror and can focus the excitationradiation onto a sample under study. The fluorescence radiation emittedby the sample can pass through the objective lens and the dichroicmirror to be focused via one or more lenses onto the photodetector. Insome embodiments in which multiple fluorescence frequenciescorresponding to fluorescence emission from multiple fluorophores withinthe sample are detected via a plurality of photodetectors (e.g.,photomultiplier tubes), each photodetector can be associated with anappropriate dichroic mirror from which it receives, via reflection,fluorescence radiation having a frequency corresponding to fluorescenceemission from one of the fluorophores. The dichroic mirror can allowfluorescence frequencies corresponding to the other fluorophores to passthrough to be detected by other downstream detectors.

In some embodiments, the fluorescence radiation emitted by the samplecan be coupled, e.g., via one or more lenses, to an optical fiber. Theoptical fiber can extend from a proximal end, which receives thefluorescence radiation, to a distal end at which the radiation exits theoptical fiber. In some embodiments, an output lens optically coupled tothe distal end of the optical fiber facilitates directing thefluorescence radiation exiting the optical fiber onto one or morephotodetectors.

The system further includes an analysis module in communication with oneor more photodetectors to receive one or more time-domain fluorescencesignals from the photodetector(s) and to reconstruct one or morefluorescence images of the sample. For example, the analysis module canprovide frequency de-multiplexing of the fluorescence signal todetermine the beat frequencies and can generate a fluorescence image ofthe sample by correlating the beat frequencies with spatial locations ofthe sample emitting fluorescence radiation modulated at those beatfrequencies. By way of example, in some embodiments, those spatiallocations can be along a horizontal dimension of the sample. In someembodiments, as the sample flows through the flow cell, fluorescenceradiation from different portions of the sample (e.g., differenthorizontal extents of the sample) is collected and analyzed to generatea two-dimensional fluorescence image of the sample.

In some embodiments, the analysis module reconstructs a fluorescenceimage of the sample by (1) obtaining a Fourier transform (e.g., FFT) ofat least a portion of the fluorescence signal to obtain frequencycomponents of the signal, which correspond to the beat frequencies, (2)for each frequency component (beat frequency), computing a measure ofamplitude of that frequency component, e.g., by obtaining the squareroot of the sum of squares of the real and imaginary parts of thatfrequency component, to provide a pixel value corresponding to alocation of the image corresponding to that beat frequency.

In some embodiments, the analysis module is configured to effect thefrequency de-multiplexing of the detected fluorescence signal bydigitizing the fluorescence signal, e.g., after amplification, andgenerating several copies of the digitized fluorescence signal, wherethe number (N) of the digitized copies corresponds to the number offrequencies associated with RF comb beams. Each digitized copy of thefluorescence signal is multiplied with sine and cosine waves having afrequency corresponding to a beat frequency equal to a differencebetween the frequencies of one of the RF comb beams and the LO beam togenerate a plurality of intermediate signals. Each intermediate signalis passed through a low-pass filter having a bandwidth equal to one halfof the frequency spacing between the RF comb frequencies. For each beatfrequency corresponding to one of the RF comb frequencies, the squareroot of the sum of the squares of the two filtered intermediate signalscorresponding to that frequency is obtained as a measure of theamplitude of an image pixel corresponding to the sample locationilluminated by the LO beam and the RF comb beam that emits fluorescenceradiation exhibiting modulation at that beat frequency.

In some embodiments, the analysis module is configured to effect thefrequency de-multiplexing of the detected fluorescence signal bygenerating several copies of the digitized fluorescence signal, wherethe number (N) of the digitized copies corresponds to the number offrequencies associated with the RF comb beams. Each copy of thedigitized fluorescence signal is filtered by passing that signal througha bandpass filter centered at a beat frequency associated with one ofthe RF comb beams. An envelope detector is employed to estimate theamplitude of each pixel corresponding to that frequency.

In some embodiments, the above system further includes a detection armfor generating a brightfield image of the sample and another detectionarm for generating a darkfield image of the sample. The brightfielddetection arm can be positioned, e.g., relative to a flow cell throughwhich the sample flows so as to receive the excitation radiation (i.e.,the combined LO and RF comb beams) in a forward direction, that is,along a direction substantially parallel to the propagation direction ofthe excitation radiation as it enters the flow cell. Further, thedarkfield detection arm can be positioned, e.g., relative to the flowcell so as to receive excitation radiation scattered by the sample alonga direction substantially orthogonal to the propagation direction of theexcitation radiation as it enters the flow cell.

In some embodiments, each of the brightfield and darkfield detectionarms includes one or more lenses for focusing radiation transmittedthrough the flow cell or radiation scattered by the sample in adirection different than the propagation direction of the excitationradiation, respectively, onto a photodetector, e.g., a photomultipliertube. In some cases, an appropriate filter, e.g., a bandpass filter, canbe placed in front of the photodetector to allow passage of desiredradiation frequencies (e.g., excitation frequencies) while blockingunwanted radiation frequencies.

In some cases, the analysis module is configured to generate a compositeimage via overlay of the brightfield and/or the darkfield and thefluorescence image.

In a related aspect, a method of flow cytometry is disclosed, whichincludes generating an excitation beam by superimposing a top-hat-shapedlaser beam (LO beam) with a plurality of laser beams that areradiofrequency shifted from one another (RF comb beams), wherein theexcitation beam is capable of eliciting fluorescence from at least onefluorophore, and directing the excitation beam onto a sample so as toexcite said fluorophore, if present in the sample, to cause it to emitfluorescence radiation. The superposition of the LO beam and the RF combbeams results in spatial encoding of a plurality of beat frequenciescorresponding to frequency differences between the RF comb beams and theLO beam. The fluorescence radiation is detected and frequencyde-multiplexed so as to generate a fluorescence image of the sample.

In some embodiments, a brightfield image and a darkfield image of thesample are generated, which can supplement the information provided bythe fluorescence image. In some embodiments, the above system furtherincludes a subsystem for performing fluorescence lifetime measurement ofthe fluorophores at multiple spatial locations on the sample to form afluorescence lifetime image. The subsystem includes a photodetector fordetecting the combined excitation beam and generating an excitationsignal in response to said detection. The analysis module is incommunication with the photodetector to receive the detected excitationsignal and to de-multiplex the beat frequencies associated with theexcitation signal to determine the phase of each of the beat frequenciesat each spatial location on the sample. The analysis module is furtherconfigured to determine the phase of each of the beat frequenciesassociated with the detected fluorescence signal and to compare thephase of each frequency associated with the combined excitation beamwith the phase of the respective beat frequency associated with thefluorescence signal to perform fluorescence lifetime measurement atmultiple spatial locations on the sample.

In a related aspect, a system for performing flow cytometry isdisclosed, which comprises a laser for generating a laser beam having afrequency suitable for exciting at least one fluorophore, anacousto-optic deflector (AOD), preferably a single acousto-opticdeflector, configured to receive said laser beam, a radio-frequencygenerator for applying one or more drive signals to said AOD, and acontroller for controlling said radio-frequency generator so as toprovide three operational modes. The operational modes include a firstoperational mode in which said controller effects the frequencygenerator to apply concurrently a plurality of radiofrequency drivesignals to said AOD so as to generate a plurality of radiofrequencyshifted beams for concurrently illuminating a plurality of spatiallocations of a sample, a second operational mode in which saidcontroller effects the frequency generator to successively apply aplurality of radiofrequency drive signals to said AOD to illuminate thesample with a plurality of radiofrequency shifted beams at differenttimes, and a third operational mode in which said controller effects thefrequency generator to apply a single radiofrequency drive signal tosaid AOD to illuminate the sample with a beam at a single frequency. Incertain embodiments, the spatial locations illuminated by theradiofrequency shifted beams are positioned along a single dimension,such that the radiofrequency shifted beams irradiate two or morepositions along a single plane. For example, the radiofrequency shiftedbeams may illuminate spatial locations along a plane that is orthogonalto the longitudinal axis of the flow stream. Each radiofrequency shiftedbeam may be spaced apart from each other at the sample (e.g., at thesurface of a flow stream) by 0.001 μm or more, such as by 0.005 μm ormore, such as by 0.01 μm or more, such as by 0.05 μm or more, such as by0.01 μm or more, such as by 0.05 μm or more, such as by 0.1 μm or more,such as by 0.5 μm or more, such as by 1 μm or more, such as by 2 μm ormore, such as by 3 μm or more, such as by 5 μm or more, such as by 10 μmor more, such as by 15 μm or more, such as by 25 μm or more andincluding by 50 μm or more.

In a related aspect, the system can include an optical element forreceiving one of the radio-frequency shifted beams (herein “localoscillator (LO) beam”) and directing said LO beam along a propagationpath different than propagation paths of the other frequency shiftedbeams (herein “RF comb beams”). The system can further include a top-hatbeam shaper for imparting a top-hat intensity profile to the LO beam andone or more optical elements for combining the top-hat shaped LO beamwith the RF comb beams in the first operational mode to form a compositeexcitation beam for illuminating the sample.

Further understanding of various aspects of the invention can beobtained by reference to the following detailed description inconjunction with the associated drawings, which are described brieflybelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a system in accordance with an embodimentof the invention,

FIG. 2A is a schematic, exemplary profile of a Gaussian beam in a planeperpendicular to the beam's propagation direction,

FIG. 2B is a schematic, top-hat beam profile obtained by passing theGaussian beam shown in FIG. 2A through a top-hat beam shaper andfocusing the output beam of the beam shaper,

FIG. 3 schematically depicts components of an exemplary top-hat beamshaper,

FIG. 4 schematically depicts cross-sectional beam profiles of aplurality of RF comb beams,

FIG. 5 schematically depicts superposition of the RF comb beams depictedin FIG. 4 and an LO beam having a top-hat beam profile,

FIG. 6 schematically depicts the combined beam shown in FIG. 5illuminating a sample under analysis,

FIG. 7 schematically depicts exemplary energy levels of a hypotheticalfluorophore,

FIG. 8 schematically depicts an absorption curve corresponding to thehypothetical fluorophore of FIG. 7,

FIG. 9A schematically depicts a detection system according to anembodiment of the present teachings, which includes an optical fiber fortransmission of fluorescence radiation,

FIG. 9B schematically depicts another detection system according to anembodiment of the present teachings in which fluorescence radiationpropagates through free space to reach a plurality of photodetectors,

FIG. 9C schematically depicts a brightfield and a darkfield imagegeneration arms for use in some embodiments of the present teachings,

FIG. 9D schematically depicts a detection system for use in someembodiments of the present teachings, which includes a detection arm forgenerating a brightfield image and a detection arm which integrates thecapabilities for the detection of excitation radiation scattered from asample as well as fluorescence radiation emitted by the sample,

FIG. 10 schematically depicts that a fluorescence signal generated by aphotodetector in an embodiment of a system according to the presentinvention can be amplified by an amplifier and the amplified signal canbe analyzed by an analysis module to construct a fluorescence image of asample under analysis,

FIGS. 11A and 11B depict various steps in a method according to anembodiment of the present invention for analysis of fluorescence signalobtained by illuminating a sample with a combined beam composed of aplurality of RF comb beams and a top-hat profiled LO beam,

FIG. 12 schematically depicts selected components of an exemplaryhardware implementation of an analysis module according to an embodimentof the present invention,

FIGS. 13A and 13B depict various steps in another method according to anembodiment of the invention for analysis of fluorescence signal obtainedby illuminating a sample with a combined beam composed of a plurality ofRF comb beams and a top-hat profiled LO beam,

FIGS. 14A and 14B depict various steps in yet another method accordingto an embodiment of the invention for analysis of fluorescence signalobtained by illuminating a sample with a combined beam composed of aplurality of RF comb beams and a top-hat profiled LO beam,

FIG. 15A schematically depicts illumination of a sample by a top-hatprofiled beam at a single excitation frequency,

FIG. 15B is a schematic view of a system according an embodiment of thepresent teachings that allows for fluorescence lifetime measurements andfluorescence lifetime imaging,

FIG. 16A is a scatter plot of darkfield intensity versus brightfieldintensity of polystyrene beads stained with 8 discrete level offluorescence dyes obtained by using a cytometry system in accordancewith an embodiment of the present teachings,

FIG. 16B shows a scatter plot of red fluorescence (PE) v. greenfluorescence (FITC) emitted by a plurality of the polystyrene beads,where the data was obtained by using the rectangular section of the plotshown in FIG. 16A as a gate,

FIGS. 16C and 16D are histograms corresponding to the data shown in FIG.16B,

FIG. 17A shows brightfield, darkfield and fluorescence images of asample containing fixed peripheral blood leukocytes stained withanti-CD45-FITC, and propidium iodide obtained using a cytometry systemaccording to the present teachings, where the sample also contained afraction of live HeLa cells that were stained with Calcein-AM,

FIG. 17B is a scatter plot in which population A represents leukocytesand population B represents HeLa cells in the sample for which FIG. 17Aprovides images,

FIG. 18A shows brightfield, darkfield and fluorescence images of asample containing fixed peripheral blood leukocytes stained withanti-CD45-FITC and propidium iodide obtained using a cytometry systemaccording to an embodiment of the present teachings, where the samplewas spiked with a small fraction of fixed MCF-7 cells stained withanti-EpCAM-FITC and propidium iodide, and

FIG. 18B is a scatter plot in which population A represents leukocytesand population B represents MCF-7 cells in the sample for which FIG. 18Aprovide images.

DETAILED DESCRIPTION

The present teachings relate generally to methods and systems forperforming fluorescent analysis of samples. As discussed below, in someembodiments, a system according to the present teachings can operate inthree operational modes for performing cytometry. Various terms usedbelow to describe the present teachings have their ordinary meanings inthe art, unless stated otherwise. For example, the term “fluorophore” isused herein consistent with its customary meaning in the art to refer toa fluorescent chemical compound that can emit radiation in response toillumination by excitation radiation.

The terms “cytometry” and “flow cytometry” are also used consistent withtheir customary meanings in the art. In particular, the term “cytometry”can refer to a technique for identifying and/or sorting or otherwiseanalyzing cells. The term “flow cytometry” can refer to a cytometrictechnique in which cells present in a fluid flow can be identified,and/or sorted, or otherwise analyzed, e.g., by labeling them withfluorescent markers and detecting the fluorescent markers via radiativeexcitation. The terms “about” and “substantially” are used herein todenote a maximum variation of 10%, or 5%, with respect to a propertyincluding numerical values.

FIG. 1 schematically depicts a system 10 for performing cytometryaccording to an embodiment of the present teachings, which can beoperated in three operational modes. As discussed in more detail below,in one operational mode, a sample under study can be illuminatedconcurrently with a plurality of excitation frequencies, each of whichcan be obtained, e.g., by shifting the central frequency of a laserbeam. More specifically, a plurality of sample locations can beconcurrently illuminated by a laser beam that is generated by mixing areference laser beam (herein also referred to as a local oscillatorbeam) with a plurality of radiofrequency-shifted laser beams such thateach sample location is illuminated by the reference beam and one of theradiofrequency-shifted beams to excite a fluorophore of interest at thatlocation, if present. In some embodiments, the reference beam can itselfbe generated via radiofrequency shifting of a laser beam. Thus, eachspatial location of the sample can be “tagged” with a different beatfrequency corresponding to a difference between the frequency of thereference beam and that of one of the radiofrequency-shifted beams. Inother words, the fluorescence radiation emitted by the fluorophore willspatially encode the beat frequencies. The fluorescence emission can bedetected and its frequency components can be analyzed to construct afluorescence image of the sample.

In another operational mode, a sample can be illuminated successivelyover a time interval by a laser beam at a plurality of excitationfrequencies. In some such embodiments, the excitation frequencies can beobtained by applying a time-varying drive signal to an acousto-opticdeflector (AOD), which receives a laser beam. In many embodiments, thelaser beam has a frequency in the hundreds of terahertz (THz) range,e.g., in a range of about 300 THz to about 1000 THz. The drive signalapplied to the AOD is typically in the radiofrequency range, e.g., in arange of about 10 MHz to about 250 MHz. The passage of the laser beamthrough the AOD generates a plurality of diffracted beams, eachcorresponding to a different diffraction order. While the zero^(th)diffracted beam exhibits no frequency shift relative to the frequency ofthe input laser beam, the higher-order diffracted beams exhibit afrequency shift relative to the frequency of the input laser beamcorresponding to the frequency of the drive signal or a multiplethereof. In some embodiments, the first order diffracted beam having afrequency corresponding to the frequency of the input laser beam shiftedby the drive signal is employed as the excitation beam for exciting afluorophore of interest, if present in a sample under analysis. As thedrive signal varies over time, the frequency and angular shift of thefirst-order diffracted beam also varies, thereby allowing theillumination of the sample at different excitation frequencies atdifferent locations. The fluorescence emission, if any, from eachilluminated location can be collected and analyzed to construct afluorescence image of the sample.

In yet another operational mode, the system 10 can be operated toilluminate a plurality of locations of a sample concurrently by a singleexcitation frequency, which can be generated, e.g., by shifting thecentral frequency of a laser beam by a radiofrequency. For example, ahorizontal extent of the sample can be illuminated by a laser beam at asingle excitation frequency. The detected fluorescence radiation can beused to analyze the fluorescence content of the sample, e.g., acell/particle.

Thus, one advantage of system 10, among others discussed below, is thatit provides significant flexibility in obtaining fluorescence emissiondata in different modes without a need to utilize different instrumentsor to make any mechanical modifications to the system when switchingbetween different operational modes.

In certain embodiments, systems include one or more light sources. Insome instances, the light source is a narrow band light source,including but not limited to a narrow wavelength LED, laser or abroadband light source coupled to one or more optical bandpass filters,diffraction gratings, monochromators or any combination thereof which incombination produces a narrow band of illuminating light. In certaininstances, the light source is a single wavelength laser, such as asingle wavelength diode laser (e.g., a 488 nm laser). In someembodiments, the subject systems include a single light source (e.g., alaser). In other embodiments, the subject systems include two or moredifferent light sources, such as 3 or more different light sources, suchas 4 or more different light sources and including 5 or more differentlight sources. For example, systems may include a first light source(e.g., laser) outputting a first wavelength and a second light sourceoutputting a second wavelength. In other embodiments, systems include afirst light source outputting a first wavelength, a second light sourceoutputting a second wavelength and a third light source outputting athird wavelength.

Each light source may have a wavelength which ranges from 300 nm to 1000nm, such as from 350 nm to 950 nm, such as from 400 nm to 900 nm andincluding from 450 nm to 850 nm. In certain embodiments, the lightsource has a wavelength that corresponds to an absorption maximum of oneor more fluorophores (as described below). For example, the light sourcemay output light having a wavelength that is in the range of one or moreof 280-310 nm, 305-325 nm, 320-350 nm, 340-375 nm, 370-425 nm, 400-450nm, 440-500 nm, 475-550 nm, 525-625 nm, 625-675 nm and 650-750 nm. Incertain embodiments, each light source outputs light having a wavelengththat is selected from 348 nm, 355 nm, 405 nm, 407 nm, 445 nm, 488 nm,640 nm and 652 nm.

The system 10 includes a laser radiation source 12 generating a laserbeam 14. By way of example, the laser beam can have a frequency in arange of about 1000 THz to about 300 THz, corresponding to a vacuumwavelength in a range of about 300 nm to about 1000 nm. The beamdiameter of the laser beam (e.g., the beam waist when a Gaussian laserbeam is employed) can be, for example, in a range of about 0.1 mm toabout 10 mm. Without any loss of generality, in this embodiment thelaser 12 emits radiation at a wavelength of 488 nm with a beam diameterof about 1 mm.

The frequency of the laser beam can be selected based on a particularapplication(s) for which the system is intended. Specifically, asdiscussed in more detail below, the laser frequency can be suitable forexciting an electronic transition of a fluorophore of interest, e.g.,via absorption of the radiation, so as to cause the fluorophore to emitfluorescence radiation at a lower frequency. A variety of laser sourcescan be employed. Some examples of such laser sources include, withoutlimitation, Sapphire 488-SF, marketed by Coherent, Inc. of Santa Clara,Calif. U.S.A., Genesis MX-488-1000-STM (Coherent, Inc.), OBIS 405-LX(Coherent, Inc.), Stadus 405-250 marketed by Vortran Laser Technology,Inc. of Sacramento, Calif. U.S.A., and LQC-660-110 of NewportCorporation of Irvine, Calif. U.S.A. Without any loss of generality, inthe present embodiment the laser beam is assumed to have a Gaussianintensity profile in a plane perpendicular to its propagation direction.

A mirror 16 receives the laser radiation beam 14 and directs the laserbeam via reflection to an acousto-optic deflector (AOD) 18. In thisembodiment, the AOD 18 is mounted on an adjustable post holder mount (A)that allows rotation of the AOD about an axis perpendicular thepropagation direction of the beam 14. A direct digital synthesizer (DDS)20 operating under control of a controller 21 can apply one or moredrive signals to the AOD 18. By way of example, in some embodiments,these drive signals can span a frequency range of about 50 MHz to about250 MHz. For example, the drive signals applied to the AOD may from 55MHz to 225 MHz, such as from 60 MHz to 200 MHz, such as from 65 MHz to175 MHz, such as from 70 MHz to 150 MHz and including from 75 MHz to 125MHz. In some embodiments, the drive signals may be separated from oneanother by a frequency in a range of about 0.1 MHz to about 4 MHz. Forexample, the drive signals may be separated from one another by afrequency of from about 0.2 MHz to about 3.9 MHz, such as from about 0.3MHz to about 3.8 MHz, such as from about 0.4 MHz to about 3.7 MHz, suchas from about 0.5 MHz to about 3.6 MHz and including from about 1 MHz toabout 3.5 MHz. In this embodiment, an electronic power amplifier 21′amplifies the radiofrequency signals generated by the DDS 20 forapplication to the AOD 18.

In the operational mode in which a sample is illuminated concurrentlywith a plurality of excitation frequencies, the RF comb generator 20applies a plurality of RF drive signals concurrently to the AOD 18. Byway of example, the number of simultaneously applied RF drive signalscan be in a range of about 20 to about 200. The interaction of the laserbeam and the drive signals results in generation of a plurality ofangularly separated laser beams each having a frequency shiftcorresponding to one of the drive signals relative to the frequency ofthe laser beam generated by the laser 12. Without being limited to anyparticular theory, in an AOD, a piezoelectric transducer can generateradiofrequency phonons in a crystal, e.g., a quartz crystal, and thescattering of the optical photons of the laser beam by suchradiofrequency phonons can result in the generation of thefrequency-shifted laser beams. One of these frequency-shifted beams 22is herein referred to as a “local oscillator” (LO) beam and theremainder of the frequency shifted beams 24 are herein referred to as“RF comb beams.” The angular separation of the frequency shifted beamscan be, for example, in a range of about 1 milliradians to about 100milliradians. For example, the angular separation of the frequencyshifted beams may range from 2 milliradians to about 95 milliradians,such as from 3 milliradians to about 90 milliradians, such as from 4milliradians to about 85 milliradians, such as from 5 milliradians toabout 80 milliradians and including from 10 milliradians to about 75milliradians.

The LO and the RF comb beams pass through a lens 26, which is in thisembodiment a positive lens with a focal length of about 50 mm. Afterpassage through the lens 26, the LO laser beam is intercepted by amirror 28, which redirects the LO beam in a different direction (in thisembodiment in a direction substantially orthogonal to the originalpropagation direction of the LO beam). The mirror 28 is positionedrelative to the RF comb beams such that these beams miss the mirror 28and propagate to a lens 30 (which in this embodiment has a focal lengthof 200 mm). In this manner, the LO beam and the RF comb beams aredirected along different propagation directions. The use of the pickoffmirror 28 in a manner disclosed above allows utilizing a single AOD togenerate both the LO beam and the RF comb beams and combining them in amanner discussed below to generate an excitation beam for illuminating asample. The use of a single AOD, rather than multiple AODs (e.g., twoAODs, one for generating the LO beam and the other for generating the RFcomb beams), simplifies the design of the system and further allowsefficient use of the system in multiple distinct operational modes, asdiscussed in more detail below.

In some embodiments, the beam profile of the LO beam is modified beforerecombining with the RF comb beams. For example, the beam profile of theLO beam may be adjusted (increased or decreased) in spatial dimension,beam shape, intensity, spatial distribution of beam, or any combinationthereof. In certain embodiments, the spatial dimensions of the beamprofile of the LO beam are modified. For example, the beam profile maybe adjusted to elongate the beam profile in one or more dimensions, suchas along an axis that is orthogonal to the longitudinal axis of a flowstream. In one example according to these embodiments, the spatialdimension (e.g., in one or more dimensions) of the beam profile may beincreased by 1% or more, such as by 2% or more, such as by 3% or more,such as by 5% or more, such as by 10% or more, such as by 25% or more,such as by 50% or more, such as by 75% or more, such as 90% or more,such as by 1.5-times or more, such as by 2-times or more, such as by3-times or more and including by 5-times or more. In another exampleaccording to these embodiments, the spatial dimension (e.g., in one ormore dimensions) of the beam profile may be decreased by 1% or more,such as by 2% or more, such as by 3% or more, such as by 5% or more,such as by 10% or more, such as by 25% or more, such as by 50% or more,such as by 75% or more, such as 90% or more, such as by 1.5-times ormore, such as by 2-times or more, such as by 3-times or more andincluding by 5-times or more.

In other embodiments, the beam shape of the LO beam is modified. Forexample, the beam shape may be modified to elongate the beam profile inone or more dimensions. In certain instances, the beam shape of the LObeam is elongated in a plane perpendicular to the propagation of thedirection of the LO beam. In certain embodiments, the shape of the LObeam profile is changed from a circular beam profile to an oval beamprofile that is elongated in an axis orthogonal to the longitudinal axisof the flow stream. In other embodiments, the shape of the LO beamprofile is changed from a circular beam profile to a rectangular beamprofile that has a long dimension in an axis orthogonal to thelongitudinal axis of the flow stream.

In still other embodiments, the intensity of the LO beam is modified.For example, the intensity of the LO beam may be increased, such as by1% or more, such as by 2% or more, such as by 3% or more, such as by 5%or more, such as by 10% or more, such as by 25% or more, such as by 50%or more, such as by 75% or more, such as 90% or more, such as by1.5-times or more, such as by 2-times or more, such as by 3-times ormore and including by 5-times or more. In other embodiments, theintensity of the LO beam is decreased, such as by 1% or more, such as by2% or more, such as by 3% or more, such as by 5% or more, such as by 10%or more, such as by 25% or more, such as by 50% or more, such as by 75%or more, such as 90% or more, such as by 1.5-times or more, such as by2-times or more, such as by 3-times or more and including by 5-times ormore. In certain embodiments, the intensity of the LO beam is modifiedto match the intensity of the RF comb beams. For example, the LO beammay have an intensity that differs from the intensity of the RF combbeams by 10% or less, such as by 9% or less, such as by 8% or less, suchas by 7% or less, such as by 6% or less, such as by 5% or less, such asby 4% or less, such as by 3% or less, such as by 2% or less, such as by1% or less, such as by 0.01% or less and including where the intensityof the LO beam differs from the RF comb beams by 0.001% or less. Incertain instances, the intensities of the LO beam and the RF comb beamsare identical.

In yet other embodiments, the spatial distribution of the beam profilemay also be modified. For example, the LO beam may be modified such thatthe intensity of the LO beam is no longer Gaussian in one or moredimensions. For example, the LO beam may be modified to have a Gaussiandistribution along a first axis that is parallel to the longitudinalaxis of the flow stream and non-Gaussian along a second axis that isorthogonal to the longitudinal axis of the flow stream.

Any beam shaping protocol may be employed to modify the beam profile ofthe LO beam, including but not limited to refractive and diffractivebeam shaping protocols. In some embodiments, the LO beam is modified bya top-hat beam shaper.

In this embodiment, the LO beam propagates to another positive lens 32(which in this embodiment has a focal length of about 200 mm). Thecombination of the lens 26 and the lens 32 magnifies and collimates theLO beam in order to appropriately fill the back aperture of a top-hatbeam shaper 34. More specifically, the LO beam 22 passes through thelens 32 and is reflected by mirrors 33 and 35 to the top-hat beam shaper34.

The top-hat beam shaper 34 shapes the phase front of the Gaussian LObeam to enable formation of a top-hat intensity profile. Morespecifically, the LO laser beam 22′ exiting the top-hat beam shaper isreflected by a beam splitter 44 and is focused by lens 46 (which in thisembodiment has a focal length of 100 mm) onto an intermediate imageplane 48. The laser beam on the intermediate image plane 48 has atop-hat intensity profile along a horizontal direction in a planeperpendicular to the propagation direction of the beam. Similar to theAOD 18, in this embodiment, the beam splitter 44 is mounted on anadjustable post holder mount (B). In this embodiment, the top-hat beamshaper generates a top-hat beam profile in which the polarization ofradiation is substantially uniform along the top-hat direction of thebeam (along the horizontal direction in this embodiment).

By way of illustration, FIG. 2A schematically depicts the Gaussianintensity profile of the LO laser beam as it enters the top-hat beamshaper. As shown schematically in FIG. 2B, on the intermediate imageplane 48, the LO laser beam exhibits a beam profile that is stretched inthe horizontal direction (in a direction perpendicular to the page inthis illustration) and is substantially constant along each horizontalline extending through the profile, e.g., the horizontal line A, butvaries vertically according to a Gaussian profile.

A variety of top-hat beam shapers can be employed. By way of example,refractive optical elements having an aspherical surface or diffractiveoptical elements can be used to produce beams with appropriate spatialphase fronts, which, after focusing by a lens, will produce a top hatprofile pattern at the focal plane of the lens. Multiple form factorsexist for such top-hat beam shapers, and a variety of implementations ofthis approach are available to create the appropriate LO beam shape atthe sample in various embodiments of the present teachings. For example,U.S. Pat. No. 6,295,168 entitled “Refractive optical system thatconverts a laser beam to a collimated flat-top beam” and U.S. Pat. No.7,400,457 entitled “Rectangular flat-top beam shaper,” both of which areherein incorporated by reference in their entirety, disclose beamshaping systems that can be employed as the flat-top beam shaper in asystem according to some embodiments of the present teachings. By way ofillustration, FIG. 3 is a reproduction of FIG. 1 of U.S. Pat. No.7,400,457 (with different reference numerals) that schematically depicta beam shaping system 300 for providing a square or a rectangular beam,which includes two orthogonally disposed acylindrical lenses 302 and304. The first acylindrical lens 302 is for shaping an incident beam Aalong the X-axis and the second acylindrical lens 304 for shaping theincident beam A along the Y-axis. The two crossed acylindrical lensesare adapted to provide a resulting rectangular laser beam B having aflat-top profile along the X-axis. The input surface 302 a of theacylindrical lens 302 is a convex acylindrical surface having a variableradius of curvature that is smaller in the center of the surface andincreases smoothly toward both extremities of the lens. The secondacylindrical lens 304 is similar to the first acylindrical lens but isorthogonally disposed relative to the lens 302 in order to shape thebeam along the Y-axis. The profiles of input surfaces 302 a/304 a, andoutput surfaces 302 b/304 b of the lenses 302 and 304 can beindependently selected as a function of the X and Y-profiles of theindicent beam A and the desired intensity profile of the resultantrectangular beam B (See, e.g., columns 5 and 6 of the patent).

An example of a commercially available top-hat beam shaper that can beemployed includes, for example, DTH-1D-0.46 deg-4 mm marketed by Osela,Inc. of Lachine, Canada.

As discussed in more detail below, the use of a beam shaper to stretchthe LO beam along the horizontal direction provides a number ofadvantages. For example, it can ensure that the combination of the LObeam and the RF comb beams illuminates a plurality of sample locationswith a substantially similar illumination intensity, in order to matchthe intensities of the LO and RF comb beams across the entirety of thesample locations, thereby creating an intensity amplitude modulation ofthe fluorescence signal with high modulation depth. In absence of suchintensity matching, the imaging system may have a small view and may notutilize all of the frequencies (pixels) driving the AOD. As themodulation depth of the fluorescence signal plays an important role inthe ability of the system to reconstruct a fluorescence image of thesample, a uniformly-high modulation depth of the excitation beatfrequencies at all pixels is particularly advantageous to the operationof the system. Further, the amplitudes of electronic signals applied tothe AOD for generating the RF comb beams can be adjusted by controllingthe output of the direct digital synthesizer (e.g., by employing thecontroller 21) in order to equalize the RF comb beams such that theirintensities are equal to that of the LO beam across all spatiallocations in which the RF comb beams and the LO beam overlap. Thisfeature provides an advantage in that it ensures high modulation depthof the intensity amplitude modulation of the fluorescence radiation.

Referring again to FIG. 1, the RF comb beams 24 are imaged via thecombination of the lenses 26 and 30 onto an intermediate image plane 38.More specifically, the RF comb beams 24 pass through the lens 26 andmiss the mirror 28 to reach the lens 30, which directs the RF comb beamsvia mirrors 40 and 42 to the intermediate image plane 38.

FIG. 4 schematically depicts the distribution of an exemplary number ofRF comb beams in the intermediate image plane 38 (without loss ofgenerality, the number of RF comb beams is selected to be 6 forillustration purposes (labeled as RF1, . . . , RF6), though othernumbers can also be employed). As shown in FIG. 4, in the intermediateimage plane 38, the RF comb beams 24 are spatially separated from oneanother along the horizontal direction. In other embodiments, two ormore of the RF comb beams 24 may partially overlap. Thus, thecombination of the lenses 26 and 30 transform the angularly separated RFcomb beams into a set of spatially separated beams that span over ahorizontal extent.

Referring again to FIG. 1, as discussed above, the beam splitter 44receives the laser beam 22′ exiting the top-hat beam shaper 34 andreflects that beam to lens 46, which in turn focuses the beam on theintermediate image plane 48 in which the LO beam exhibits a top-hat beamprofile. The beam splitter also receives the RF comb beams 24 from theintermediate image plane 38 and allows the passage of the RF comb beamstherethrough. The lens 46 focuses the RF comb beams 24 onto theintermediate image plane 48 to be combined with the LO beam having atop-hat beam profile to generate a combined beam 49.

By way of illustration, FIG. 5 schematically depicts one exemplaryprofile of the combined beam 49 in a plane perpendicular to itspropagation axis. The intensity profile of the combined beam isgenerated as a superposition of the intensity profile of the top-hat LObeam (shown schematically by the square) and those of the RF comb beams24 (each shown schematically by one of the circles). As discussed inmore detail below, this superposition of the LO beam and the RF combbeams provides, along a horizontal extent, a plurality of beatfrequencies each corresponding to one spatial location along thathorizontal extent. Upon illuminating a horizontal extent of a sample,the fluorescence radiation emitted from a location of the sampleencodes, via amplitude modulation, the beat frequency associated withradiation illuminating that location.

Referring again to FIG. 1, a positive lens 50 (200-mm lens in thisembodiment) and an objective lens 52, mounted in this embodiment on anadjustable post holder mount C, form a telescope for relaying the imageat the intermediate plane 48 onto a sample flowing through a flow cell54. In this embodiment, a mirror 56 reflects the combined beam 49 to thelens 50, and a dichroic mirror 58 reflects the combined light beam afterits passage through the lens 50 toward the objective lens 52.

As shown schematically in FIG. 6, the combined beam 49 concurrentlyilluminates a plurality of spatial locations 60 of a sample 62 flowingthrough the flow cell 54. Thus, each location 60 is illuminated by theoverlap of one of the RF comb beams with a portion of the top-hat shapedLO laser beam. At these spatial locations, the radiation will excite afluorophore of interest in the sample, if present. More specifically, inthis embodiment, the LO beam and the RF comb beams excite concurrentlythe fluorophore, e.g., via causing electronic transition thereof to anexcited electronic state, at a plurality of sample locations 60.

In some embodiments, the sample can include a flowing fluid, in which aplurality of cells are entrained. In some cases, the cells can belabeled with one or more fluorescent markers (fluorophores). Someexamples of fluorescent markers include, without limitation, fluorescentproteins (e.g., GFP, YFP, RFP), antibodies labeled with fluorophores(e.g., fluorescein isothiocyanate) (FITC), phycoerythrin (PE),allophycocyanin (APC)), nucleic acid stains (e.g.,4′,6-diamidino-2-phenylindole (DAPI), SYTO16, propiedium iodide (PI)),cell membrane stains (e.g., FMI-43), and cell function dyes (e.g.,Fluo-4, Indo-1). In other cases, endogenous fluorophores present incells can be employed to elicit fluorescent radiation from the cells. Asdiscussed in more detail below, such exogenous or endogenousfluorophores undergo electronic excitation in response to theilluminating radiation and emit fluorescent radiation (typically at alower frequency than the excitation frequency), which is collected andanalyzed.

By way of illustration and without being limited to any particulartheory, FIG. 7 shows hypothetical energy levels corresponding to aground electronic state A as well as two electronic excited electronicstates B and C of a fluorophore. The fluorophore can be excited from itsground electronic state (A) to the excited electronic state (B) viaabsorption of radiation energy. The fluorophore can then relax into thelower excited state B, e.g., via a radiation-less transition mediated byvibrational modes of the fluorophore. The fluorophore can further relaxfrom the lower electronic state C to the ground state, via an opticaltransition, thereby emitting fluorescence radiation at a frequency lessthan that of the excitation frequency. It should be understood that thishypothetical example is provided only for illustration purposes, and notto indicate the only mechanism by which fluorescence radiation can beemitted.

In many cases, the fluorophore can absorb electromagnetic radiation overa range of frequencies to be excited from the ground state to theexcited electronic state. By way of illustration, FIG. 8 shows anabsorption curve for the hypothetical fluorophore discussed inconnection with FIG. 7. In one implementation of an embodiment accordingto the present teachings the LO frequency can be selected to coincidewith the frequency corresponding to the peak absorption of a fluorophoreof interest. The radiofrequency-shifted beams can have frequenciesseparated from the peak absorption by their respective beat frequencies.Typically, these frequency separations are small in comparison to theabsorption bandwidth of the fluorophore so as to avoid any degradationof the excitation frequency. By way of example and only by way ofillustration, the dashed lines A and B schematically depict thefrequency of the LO beam and one of the RF comb beams (the figures isnot drawn to scale for ease of description). The concurrent illuminationof a spatial location of the sample by both the LO laser beam and one ofthe depicted RF comb beams results in fluorescence radiation exhibitingan amplitude modulation at a beat frequency corresponding to adifference between the LO and the RF comb beam frequencies.

Again by way of illustration and without being limited to any particulartheory, the electric field applied to the fluorophore via its concurrentillumination by the LO beam and one of the RF comb beams can bemathematically defined as follows:

$\begin{matrix}{E_{com} = {{E_{RF}e^{j{({\omega_{0} + \omega_{RF}})}}} + {E_{LO}e^{j{({\omega_{0} + \omega_{LO}})}}}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$

wherein,

E_(com) denotes the electric field of the combined beam,

E_(RF) denotes the amplitude of the electric field associated with oneof the RF comb beams,

E_(LO) denotes the amplitude of the electric field associated with theLO beam,

φ₀ denotes the frequency of the laser beam generated by the laser 12,

φ_(RF) denotes the frequency shift associated with the RF comb beam, and

φ_(LO) denotes the frequency shift associated with the LO beam.

The intensity of the fluorescence radiation emitted in response to thesuperposition of the electric fields of the LO and RF comb beams wouldexhibit a modulation at a beat frequency corresponding to(ω_(RF)-ω_(LO)). Hence, the fluorescence radiation emanating from eachspatial location of the sample illuminated by superposition of the LObeam and one of the RF comb beams exhibits a modulation at a beatfrequency corresponding to the difference between the radiofrequencyshift associated with the LO beam and that associated with the RF combbeam illuminating that spatial location.

As the process of fluorescence emission requires a finite amount of time(typically 1-10 nanoseconds for common organic fluorophores), theemitted fluorescence will not exhibit a high modulation depth if theexcitation beat frequency is too high. Thus, in many embodiments, theexcitation beat frequencies are selected to be considerably less than1/τ_(f), where τ_(f) is the characteristic fluorescence lifetime of thefluorophore. In some instances, the excitation beat frequencies may beless than 1/τ_(f) by 1% or more, such as by 1% or more, such as by 2% ormore, such as by 3% or more, such as by 5% or more, such as by 10% ormore, such as by 25% or more, such as by 50% or more, such as by 75% ormore, such as 90% or more, such as by 1.5-times or more, such as by2-times or more, such as by 3-times or more and including by 5-times ormore. For example, the excitation beat frequencies may be less than1/τ_(f) by 0.01 MHz or more, such as by 0.05 MHz or more, such as by 0.1MHz or more, such as by 0.5 MHz or more, such as by 1 MHz or more, suchas by 5 MHz or more, such as by 10 MHz or more, such as by 25 MHz ormore, such as by 50 MHz or more, such as by 100 MHz or more, such as by250 MHz or more, such as by 500 MHz or more and including by 750 MHz ormore.

In embodiments, the photodetector is configured to detect light (e.g.,luminescence such as fluorescence) from the irradiated sample. Inembodiments, the photodetector may include one or more detectors, suchas 2 or more detectors, such as 3 or more detectors, such as 4 or moredetectors, such as 5 or more detectors, such as 6 or more detectors,such as 7 or more detectors and including 8 or more detectors. Any lightdetecting protocol may be employed, including but not limited toactive-pixel sensors (APSs), quadrant photodiodes, image sensors,charge-coupled devices (CCDs), intensified charge-coupled devices(ICCDs), light emitting diodes, photon counters, bolometers,pyroelectric detectors, photoresistors, photovoltaic cells, photodiodes,photomultiplier tubes, phototransistors, quantum dot photoconductors orphotodiodes and combinations thereof, among other photodetectors. Insome embodiments, photodetectors of interest are configured to detectlight that ranges from 350 nm to 1200 nm, such as from 450 nm to 1150nm, such as from 500 nm to 1100 nm, such as from 550 nm to 1050 nm, suchas from 500 nm to 1000 nm and including from 400 nm to 800 nm. Incertain embodiments, the photodetector is configured to detect light atthe emission maximum of the luminescence, such as at 395 nm, 421 nm, 445nm, 448 nm, 452 nm, 478 nm, 480 nm, 485 nm, 491 nm, 496 nm, 500 nm, 510nm, 515 nm, 519 nm, 520 nm, 563 nm, 570 nm, 578 nm, 602 nm, 612 nm, 650nm, 661 nm, 667 nm, 668 nm, 678 nm, 695 nm, 702 nm, 711 nm, 719 nm, 737nm, 785 nm, 786 nm, 805 nm.

In some embodiments, the fluorescence radiation emitted by the samplecan be collected in a variety of different ways, e.g., along an opticalpath that is perpendicular to the propagation direction of theexcitation beam. In other embodiments, the fluorescence radiation isdetected in an epi-direction.

Referring again to FIG. 1, in this embodiment, the fluorescenceradiation emitted by one or more fluorophores present in the illuminatedsample passes through the objective lens 52 and is transmitted throughthe dichroic mirror 58 to reach a photodetector 64. More specifically,in this embodiment, a lens 65 focuses the fluorescent radiationtransmitted through the dichroic mirror 58 onto a slit aperture 66. Thefluorescent radiation that is transmitted through the slit passesthrough a fluorescence emission filter 68 to reach the photodetector 64.The slit aperture 66 (or an optical filter in other embodimentsdiscussed below) disposed in front of the photodetector substantiallyallows the passage of the fluorescence radiation emitted from aparticular plane of the sample while rejecting out-of-plane fluorescenceemission. Further, the fluorescence emission filter 68, e.g., a passbandfilter, allows the passage of fluorescence radiation to thephotodetector 64 while substantially blocking the passage of radiationat other frequencies.

The photodetector 64 has sufficient RF bandwidth to detect and transmitsignals from the entire range of the beat frequencies. Some examples ofsuitable photodetectors include, without limitation, a photomultipliertube, avalanche photodiode, PIN photodiode, and a hybrid photodetector,among others. By way of example, in some embodiments, a photomultipliertube marketed by Hamamatsu Corporation can be employed (e.g., R3896,R10699, H11462). The photodetector generates a signal, e.g., an analogsignal in this embodiment, in response to the detection of the receivedfluorescence radiation.

By way of another example and with reference to FIG. 9A, thefluorescence radiation emitted by the sample in response to concurrentillumination by the LO beam and the spatially separated RF comb beamspasses through the objective lens 52 and the dichroic mirror 58 to becoupled via a lens 100 onto a multimode optical fiber 102, which extendsfrom a proximal end 102 a to a distal end 102 b. More specifically, theproximal end 102 a of the optical fiber 102 is positioned in proximityof the focal plane of the lens 100 so as to receive the fluorescentradiation. An outcoupling lens 104, coupled to the distal end 102 b ofthe optical fiber, collimates the radiation exiting the fiber.

In many cases, the excitation radiation illuminating the sample excitesmultiple fluorophores (e.g., organic fluorophores) that can have broadenough radiation absorption spectra such that the excitation frequenciesfalls within the absorption spectra of multiple fluorophores in thesample. Each fluorophore would then emit fluorescence radiation at adifferent frequency. Without loss of generality and for purposes ofillustration, in this embodiment, the detection system includes fourphotomultiplier tubes 106, 108, 110 and 112, each of which receives aportion of the collimated radiation corresponding to the fluorescenceradiation emitted by one of four fluorophores excited by the excitationradiation in the illuminated sample. More specifically, a dichroicmirror 114 reflects the fluorescence radiation emitted by one of thefluorophores at a first frequency to the photomultiplier tube 106 whileallowing fluorescence radiation at other frequencies to pass through.Another dichroic mirror 116 reflects the fluorescence radiation emittedby a different fluorophore at a different second frequency to thephotomultiplier tube 108 while allowing the rest of the radiationcontaining fluorescence radiation emitted by yet another fluorophore ata third frequency to reach a third dichroic mirror 118, which reflectsthat fluorescence radiation to the photomultiplier tube 110. Thedichroic mirror 118 allows the rest of the radiation including thefluorescence radiation emitted by a fourth fluorophore at a fourthradiation frequency to pass through to reach the photomultiplier tube112.

A plurality of bandpass filters 120, 122, 124, and 126, each centered atone of the four fluorescence frequencies, are placed in front of thephotomultiplier tubes 106, 108, 110, and 112, respectively. The signaldetected by each of the photomultiplier tubes is analyzed in a mannerdiscussed below to generate a fluorescence image at the respectivefluorescence frequency. In some embodiments, rather than using multiplephotodetectors, a single photodetector, e.g., a single photomultipliertube can be used to detect fluorescence radiation, e.g., fluorescencefrequency corresponding to emission from a single fluorophore.

In some embodiments, as the sample flows through the flow cell differenthorizontal rows of the sample are illuminated and fluorescence radiationassociated with each horizontal row is detected by one or morephotodetectors, such as the photomultipliers 106, 108, 110 and 112.

FIG. 9B schematically depicts a detection system similar to thatdiscussed above in connection with FIG. 9A except that this detectionsystem, rather than using an optical fiber, the fluorescence radiationcontaining fluorescence emission from a plurality of fluorophorespassing through the dichroic mirror 58 propagates in free space to reachthe photomultiplier tubes 106, 108, and 112. More specifically, the lens100 focuses the fluorescence radiation onto an aperture 126 disposedbetween the lenses 100 and 104, where the aperture can rejectout-of-focus radiation. The lens 104 collimates the radiation passingthrough the aperture, where the collimated radiation is distributedamong the photomultiplier tubes in a manner discussed above inconnection with FIG. 9A.

In some embodiments, the system 10 can be configured to provide adarkfield image and a brightfield image of the sample (of the flow cellin absence of the sample) using the excitation radiation. By way ofexample, FIG. 9C schematically depicts an embodiment of the system 10that includes two detection arms 200 and 202 for detecting,respectively, a darkfield image and a brightfield image of the sample.

More specifically, the detection arm 200 is positioned perpendicular tothe propagation of the excitation radiation so as to receive a portionof the excitation radiation that is scattered by the sample flowingthrough the flow cell. The detection arm 200 includes two lenses 204 and206 that collectively direct at least a portion of the excitationradiation scattered by the sample into a solid angle subtended by thelens 204 onto a photomultiplier tube 208. More specifically, the lens204 collimates the received scattered radiation and the lens 206 focusesthe collimated scattered radiation onto the photomultiplier tube 208. Inthis embodiment, an appropriate bandpass filter 210 is disposed in frontof the photomultiplier tube 208 to allow the passage of radiation havingthe desired frequency to the photomultiplier tube 208 while blockingradiation at unwanted frequencies. The output of the photomultipliertube 208 can be processed in a manner known in the art, e.g., by ananalysis module such as that discussed below to generate a darkfieldimage.

The detection arm 202 in turn includes two lenses 212 and 214, where thelens 212 collimates the excitation radiation exiting the flow cell in aforward direction (substantially along the propagation direction of theexcitation radiation entering the flow cell 54) and the lens 214 focusesthe collimated radiation onto a photodetector 216. An appropriate filter218, e.g., a bandpass filter, disposed in front of the photodetectorallows transmission of the excitation frequencies to the photodetector216 while substantially blocking other radiation frequencies. The outputof the photodetector 216 can be processed in a manner known in the artto generate a brightfield image of the flow cell.

Thus, the detection arm 200 detects the excitation radiation that isscattered by the fluid flowing through the cell, and the detection arm202 detects the excitation radiation that is transmitted through theflow cell. When no fluid is flowing through the flow cell, the signaldetected by the photomultiplier tube 208 is low and the signal detectedby the photodetector 216 is high as there is little scattering of theexcitation radiation passing through the flow cell and hence a largepercentage, and in some cases all, of the excitation radiation istransmitted through the flow cell. In contrast, the flow of a fluidsample through the flow cell can cause the signal generated by thephotomultiplier tube 208 to increase due to scattering of a portion ofthe excitation radiation by the sample, and the signal generated by thephotodetector 216 decreases as the level of the excitation radiationtransmitted through the flow cell decreases.

By way of further example and with reference to FIG. 9D, in oneembodiment of a system according to the present teachings, a detectionarm 220 a positioned relative to the flow cell 54 in a directionsubstantially orthogonal to the propagation direction of the excitationradiation includes photodetectors for detecting both the fluorescenceradiation emitted by a plurality of fluorophores in the sample as wellas excitation radiation that is scattered by the sample. Morespecifically, the detection arm 220 includes lenses 222 and 224 thatdirect the fluorescence radiation as well as the scattered excitationradiation onto an aperture 226, which rejects unfocused radiation. Alens 228 collimates the radiation passing through the aperture. Adichroic mirror 230 reflects the portion of the radiation at theexcitation frequencies onto a photomultiplier tube 232 for detection ofa darkfield image while allowing fluorescence radiation to pass through.An appropriate filter 232 a, e.g., a bandpass filter, disposed in frontof the photomultiplier tube 232 allows the passage of radiation atexcitation frequencies to the photomultiplier tube 232 while blockingunwanted radiation frequencies. Another dichroic mirror 234 reflectsfluorescence radiation emitted by a fluorophore at a first frequencyonto a photomultiplier tube 236 while allowing the passage offluorescence radiation emitted by other fluorophores at otherfrequencies. Another dichroic mirror 238 reflects fluorescence radiationemitted by another fluorophore at a second frequency onto aphotomultiplier tube 240 while allowing the passage of fluorescenceradiation emitted by yet another fluorophore at a third frequency, whereit is detected by the photomultiplier tube 242. Similar to the previousembodiments, a plurality of filters 236 a, 240 a, and 242 a are disposedin front of the photomultiplier tubes 236, 240, and 242, respectively,to allow the transmission of radiation at desired frequencies whilesubstantially blocking unwanted radiation frequencies.

With continued reference to FIG. 9D, this implementation of a systemaccording to the present teachings further includes another detectionarm 220 b for generating a brightfield image, e.g., in a mannerdiscussed in connection with FIG. 9C. More specifically, the detectionarm 202 includes two lenses 212 and 214 that focus the light onto aphotodetector 216 for generating a brightfield image of the excitationradiation. A filter 218, e.g., a bandpass filter, is placed in front ofthe photodetector 216 to allow the passage of the excitation radiationto the detector while rejecting unwanted radiation frequencies.

Referring again to FIG. 1 as well as FIG. 10, in this embodiment, atransimpedance amplifier 70 can be coupled to the output ofphotodetector 64 (or each of the photodetectors discussed in connectionwith FIGS. 9A-9D) to amplify the signal generated by the photodetector.A data analysis unit 72 (herein also referred to as an analysis moduleor an analyzer) receives the amplified signal and analyzes the signal togenerate a fluorescence image of the sample. The data analysis unit 72can be implemented in hardware, firmware, and/or software. By ofexample, a method for analyzing the detected fluorescence data can bestored in a read-only-memory (ROM) unit of the analysis module to beaccessed under the control of a processor to analyze the receivedfluorescence signal.

As discussed in more detail below, the analysis method determines thefrequency components of the time-varying photodetector's output andconstructs a fluorescence image of the sample based on those frequencycomponents. A variety of methods for determining the frequency contentof the photodetector's output can be employed. Some examples of suchsuitable methods include, without limitation, Fourier transform, lock-indetection, filtering, I/Q demodulation, homodyne detection, andheterodyne detection.

By way of example, FIGS. 11A and 11B show exemplary analysis steps thatcan be performed by the analysis module 72 to generate a fluorescenceimage of the sample. In step (1), the analog amplified signal isdigitized to generate digitized fluorescence data. In step (2), anappropriate portion (length) of the digitized data is selected foranalysis. For example, the fluorescence data corresponding to anilluminated row of the sample (herein also referred to as a frame) canbe chosen for analysis. Alternatively, a portion of a data frame can beselected for analysis.

In step (3), a Fourier transform of the selected data is performed. Byway of example, in some embodiments, a Fast Fourier Transform (FFT) ofthe data is performed to determine frequency components of the data. Insome such embodiments, the bins of the FFT can correspond to thefrequencies chosen for data acquisition. For example, for a 256 MHzsampling rate, 256 samples can yield frequency bins that are separatedfrom one another by 1 MHz, e.g., from DC to 128 MHz. The FFT analysisprovides frequencies corresponding to the beat frequencies at which theemitted fluorescence emission exhibits amplitude modulation.

With continued reference to FIGS. 11A and 11B, in this embodiment, instep (4), a measure of the amplitude of each frequency component presentin the FFT data is computed by obtaining the square root of the sum ofsquares of the real and imaginary components of that frequencycomponent. As each frequency component corresponds to one of the beatfrequencies employed to elicit the fluorescence radiation from aparticular location of the sample, the measure of the amplitude of thefrequency component can provide a pixel value for a location associatedwith that frequency component along a horizontal row of the sample. Inthis manner, pixel values for an image of a horizontal row of the samplecan be determined. The above steps can be repeated for fluorescence dataobtained for each horizontal row of the sample as the sample flowsthrough the flow cell in a vertical direction. The pixels values can beused to construct a fluorescence image (step 5).

As noted above, the analysis module 72 can be implemented in hardware,firmware and/or software using techniques known in the art and inaccordance with the present teachings. By way of example, FIG. 12schematically depicts an exemplary implementation of analyzer 72, whichincludes an analog-to-digital converter 74 for receiving the amplifiedfluorescence signal from the amplifier 70 and digitizing that signal togenerate digitized fluorescence data. The analysis module furtherincludes a central processing unit (CPU) 76 for controlling theoperation of the analysis module, including performing calculations andlogic operations. The analysis module also includes ROM (read onlymemory) 78, RAM (random access memory) 80 and permanent memory 82. Acommunications bus 84 facilitates communication among various componentsof the analysis module, including communications between the CPU 76 andother components. The memory modules can be used to store instructionsfor analyzing the fluorescence data and the analysis results. By way ofexample, in some embodiments, instructions for data analysis, e.g.,instructions for performing the above steps discussed in connection withFIGS. 11A and 11B, can be stored in the ROM 78. The CPU can employinstructions stored in ROM 78 to operate on digitized fluorescence datastored in RAM 80 to generate a fluorescence image of the sample (e.g., aone-dimensional or a two-dimensional image). The CPU can effect thestorage of the fluorescence image in permanent memory 82, e.g., in adatabase. As shown schematically in FIG. 12, the analysis module canoptionally include a graphics processing unit (GPU) 76′ for performingcalculations of pixel intensities and other quantities from the receiveddata (e.g., fluorescence data).

In some embodiments, the frequency demodulation of the output signalgenerated by the photodetector can be achieved using lock-in detectiontechniques. By way of example, with reference to FIGS. 13A and 13B, inone such embodiment, the amplified fluorescence signal is digitized(step 1) and several copies of the digitized fluorescence signal aregenerated (step 2), where the number (N) of the digitized copiescorresponds to the number of frequencies associated with the RF combbeams. Each digitized copy of the signal is multiplied with sine andcosine waves having a frequency corresponding to a beat frequency equalto a difference between the frequencies of one of the RF comb beams andthe LO beam to generate a plurality of intermediate signals (step 2).Each intermediate signal is passed through a low-pass filter (step 3),which has a bandwidth equal to one half of the frequency spacing betweenthe RF comb frequencies.

For each beat frequency corresponding to one of the RF comb frequencies(in other words, for each frequency corresponding to a spatial locationof the illuminated sample), square root of the sum of the squares of thetwo filtered intermediate signals corresponding to that frequency isobtained as a measure of the amplitude of an image pixel correspondingto the sample location illuminated by the LO beam and the RF comb beamhaving that frequency (step 4). In some embodiments, multiplefluorescence data signals corresponding to the same beat frequency(i.e., corresponding to the same sample location) can be processed in amanner discussed above and the pixel values can be averaged so as toobtain an average pixel value.

The above steps can be repeated for fluorescence data obtained for eachhorizontal row of the sample as the sample flows through the flow cellin a vertical direction. The pixels values can be used to construct afluorescence image (step 5).

The above lock-in detection method can be implemented in software,firmware and/or hardware. By way of example, in one embodiment the abovelock-in detection method can be implemented using a field programmablegate array (FPGA), particularly if more than 6 frequencies are used. Insome embodiments, a multi-frequency lock-in amplifier, such as HF2L-MFmulti-frequency amplifier marketed by Zurich Instruments of Zurich,Switzerland can be employed.

By way of further examples, in some embodiments the frequencydemodulation of the detected fluorescence signal can be achieved byemploying a bandpass filter-based image demodulation technique. Byreference to FIGS. 14A and 14B, in one embodiment of such a frequencydemodulation method, the fluorescence signal provided by thephotodetector 64 and the amplifier 70 is digitized (step 1) and severalcopies of the digitized signal are generated (step 2), where the number(N) of the digitized copies corresponds to the number of frequenciesassociated with the RF comb beams. Each copy of the digitizedfluorescence signal is filtered by passing that signal through abandpass filter centered at a beat frequency associated with one of theRF comb beams (i.e., a beat frequency associated with a particularlocation of the sample) (step 3). More specifically, each bandpassfilter is centered at one of N beat frequencies and has a bandwidth thatis equal to half of the frequency spacing between adjacent beatfrequencies.

An envelope detector at each beat frequency is employed to estimate, foreach horizontal line, the amplitude of each pixel corresponding to thatfrequency (step 4). In some cases, a plurality of pixel valuescorresponding to a pixel, obtained by processing multiple fluorescentsignals corresponding to a sample location associated with that pixel,is averaged to obtain an average pixel value. The above steps can berepeated for fluorescence data obtained for each horizontal row of thesample as the sample flows through the flow cell in a verticaldirection. The pixels values can be used to construct a one-dimensionalor a two-dimensional fluorescence image of the sample (step 5).

The analysis module can also be configured to receive and process thebrightfield and darkfield image data. For example, with reference toFIG. 9C and FIG. 10, the analysis module 72 can be further configured toreceive the darkfield and brightfield image data from photodetectors 208and 218 to generate darkfield and brightfield images. For example, withreference to FIG. 12, the instructions for generating the darkfield andbrightfield images, e.g., in a manner known in the art, can be stored inpermanent memory 82. The processor 76 can employ these instructions toprocess the received darkfield and brightfield image data to generatethe images. The analysis module can be also configured to generatecomposite images by overlaying, e.g., a fluorescence image and one orboth of the brightfield and darkfield images.

The fluorescence images as well as the brightfield and darkfield imagesgenerated by a system according to the present teachings, such as theabove system 10, can be used for a variety of different ways. Forexample, the fluorescence image can be integrated to produce a valuecomparable to the data produced by a conventional flow cytometer. Thefluorescence image can also be analyzed to determine the location offluorescent probe giving rise to that image (e.g., it can be determinedwhether the probe is the nucleus, cytoplasm, localized to organelles, oron the outside of the cell membrane). Further, in some applications,multiple fluorescent images obtained by detecting different fluorescentbands, all of which taken from the same cell, can be used to determinethe degree of co-localization of multiple fluorescent probes within acell. Additionally, the analysis of cell morphology, cell signaling,internalization, cell-cell interaction, cell death, cell cycle, and spotcounting (e.g., FISH), among others, are possible using multi-colorfluorescence, brightfield, and darkfield images.

As noted above, the system 10 can be operated in at least threedifferent modes. In one mode discussed above, an LO beam and a pluralityof RF comb beams concurrently illuminate a portion of the sample (e.g.,locations disposed along a horizontal extent), and the fluorescenceradiation emitted from the illuminated locations is detected andanalyzed in order to construct a fluorescence image of the sample. Inanother operational mode, rather than applying a plurality of RF drivesignals concurrently to the AOD, a frequency ramp containing the drivesignals is applied to the AOD such that the frequency of the laser beamis changed over time from a start frequency (f₁) to an end frequency(f₂). For each drive frequency in the frequency ramp, the frequency ofthe laser beam is shifted by that drive frequency and the sample isilluminated by the frequency-shifted laser beam to elicit fluorescenceradiation from the sample. In other words, in this mode, the system isoperated to obtain fluorescence radiation from the sample byilluminating the sample successively over a temporal interval with aplurality of frequencies, which are shifted from the central laserfrequency. The frequency shift generated by the AOD is accompanied by anangular deflection such that using the same optical path, the beam isscanned across the sample at a high speed.

More specifically, in this operational mode, the RF frequencysynthesizer 10 is employed to ramp a drive signal applied to the AOD 18from a start frequency (f₁) to an end frequency (f₂). By way of example,the frequency range over which the drive signal is ramped can be fromabout 50 MHz to about 250 MHz. In some embodiments, the drive signal isramped from about 100 MHz to about 150 MHz. In this embodiment, thedrive frequency is changed over time continuously, e.g., to achieve ahigh speed. In other embodiments, the drive frequency can be changed indiscrete steps from a start frequency (f₁) to an end frequency (f₂).

The drive frequencies are chosen such that the frequency-shifted beamwould miss the mirror 28 and propagate along an optical path defined bylens 26, lens 30, mirrors 40/42, a beam splitter 44, lens 46, mirror 56,lens 50, mirror 58 and the objective lens 52 to illuminate a portion ofthe sample flowing through the sample holder. The ramp rate ispreferably fast enough so as to ameliorate and preferably prevent, anyblur in the vertical direction of a fluorescence image to be generatedbased on the emitted fluorescence radiation as the sample flows acrossthe beam. This can be achieved, for example, by matching the ramp ratewith the sample's flow speed. The laser spot size at the sample can beused to estimate appropriate rates. By way of example, for a laser spotsize of 1 micrometer, the scan time across 1 line should be 10microseconds or less for a sample flow speed of 0.1 meters per second toavoid image blur.

The fluorescence radiation emitted from the sample in response toillumination by the excitation radiation is collected and detected in amanner discussed above. Specifically, with reference to FIG. 10, thefluorescence radiation is detected by photodetector 64. The detectedfluorescence is amplified by the amplifier 70 and the amplified signalis analyzed by the analysis module 72 to reconstruct a fluorescenceimage of the sample. The reconstruction of the image is performed byassigning a horizontal pixel location to a specific time within the scanperiod from the start frequency (f₁) to the end frequency (f₂). Asopposed to analyzing the amplitude of a frequency component to obtainpixel values as in the above operational mode, the demodulation approachused in this operational mode only uses the time domain values of thedetected fluorescence signal to assign values to the pixels of theimage. The process can be repeated as the sample flows in a verticaldirection so as to obtain a two-dimensional fluorescence image of thesample.

The fluorescence radiation, if any, emitted by the sample is collectedby photodetector 64. Referring to FIG. 10, the detected fluorescenceradiation is amplified by the amplifier 70. The analysis module 72receives the amplified signal. In this operational mode, the analysismodule analyzes the fluorescence signal to determine the fluorescencecontent of the sample, e.g., a cell/particle. Since there is only onebeam exciting the sample in this operational mode, no beat frequenciesare generated in response to exciting the sample. Hence, there is noimage information in the frequency domain of the fluorescence signal.Rather, the detected fluorescence signal has image information encodedin the time domain. In this operational mode, an image can be digitallyreconstructed using the time values of the detected fluorescence signalas the horizontal pixel coordinate, and the digitized voltage values ofthe fluorescence signal as the pixel values (brightness). Each scan ofthe drive frequencies applied to the AOD produces one horizontal line(row) of the image. The image reconstruction is achieved via consecutivescans as the sample flows through the illumination area (point).

In yet another operational mode, the system 10 can be operated toilluminate a plurality of locations of a sample concurrently by a singleexcitation frequency, which can be generated, e.g., by shifting thecentral frequency of a laser beam by a radiofrequency. Morespecifically, referring again to FIG. 1, in such an operational mode asingle drive radio frequency can be applied to the AOD 18 to generate alaser beam having a frequency that is shifted relative to the laser beamentering the AOD 18. Further, the frequency-shifted laser beam exhibitsan angular shift relative to the laser beam entering the AOD such thatthe radiofrequency laser beam is intercepted and reflected by the mirror28 towards the top-hat beam shaper 34 via lens 32 and mirrors 33 and 35.The beam exiting the top-hat beam shaper is reflected by the beamsplitter 44 and is focused by the lens 46 onto the intermediate imageplane 48. In this plane, as shown schematically in FIG. 15A, the laserbeam 1000 shows a stretched profile along the horizontal direction.

The horizontally-stretched laser beam is reflected by the mirror 56 tothe positive lens 50. After passage through the lens 50, the laser beamis reflected by the mirror 58 to the objective lens 52. As discussedabove, the positive lens 50 and the objective lens 52 form a telescopefor relaying the top-hat profiled laser beam from the intermediate imageplane 48 onto a sample flowing through the flow cell 54.

The horizontally-stretched laser beam illuminates a horizontal extent ofthe sample to excite a fluorophore of interest, if present in thesample, along that horizontal extent. Thus, in this operational mode,unlike the first operational mode in which a plurality of horizontallocations of the sample is illuminated at different excitationfrequencies, a plurality of horizontal locations of the sample isilluminated at the same excitation frequency. This operational mode doesnot enable a user to obtain an image of cells or particles that flow by.However, in this operational mode, a higher optical power can typicallybe applied to the sample than in the other two operational modes, whichcan be useful for obtaining a higher signal-to-noise ratio data ifimages are not required. This operational mode is accessible by merelyaltering the electronic signal driving the acousto-optic deflector,without a need to make any mechanical changes to the system.

Thus, the system 10 can be operated in three distinct operational modeto elicit fluorescence radiation from a sample.

In some embodiments, fluorescence lifetime measurements can be performedat each spatial position on the sample, e.g., by comparing the phase ofthe beats of each of the radiofrequency-shifted and local oscillatorbeams with the phase of a respective radiofrequency component in thedetected fluorescence signal. By way of example, FIG. 15B shows a system10′, a modified version of the system 10 discussed above, that allowsfor such fluorescence lifetime measurements (certain components shown inFIG. 1 are not depicted in this figure for brevity). Specifically, aportion of the RF comb beams incident on the beam splitter 44 isreflected by the beam splitter onto a convergent lens 400 (by way ofillustration in this embodiment the lens 400 has a focal length of 200mm, though other focal lengths can also be used). The lens 400 focusesthat portion of the RF comb beams onto a photodiode 402, which detectsthe excitation beam. The output of the photodiode 402 can be received bythe analysis module 72 (See, FIG. 10). The analysis module can providefrequency de-multiplexing of the excitation beam, e.g., using one of thede-modulation techniques discussed above and determine the phase of eachradio frequency component in the excitation beam. This can provide, foreach radiofrequency component in the detected fluorescence signal, areference phase with which the phase of that radiofrequency componentcan be compared. For example, the real and imaginary components of anFFT of the excitation signal or the I and Q components of lock-in typedemodulation can be employed. Alternatively, the output of the detectordetecting the brightfield image of the sample/flow cell can be used toobtain reference phases with which the phases of the fluorescence beatfrequencies can be compared.

More specifically, the analysis module 72 can provide frequencyde-multiplexing of the detected fluorescence signal, e.g., in a mannerdiscussed above. As will be appreciated by one skilled in the art, foreach beat frequency in the fluorescence signal, the phase of theradiofrequency component can be compared with the respective referencephase of the excitation beam to obtain spatially-resolved fluorescencelifetime measurements and a fluorescence lifetime image.

In certain embodiments, the subject systems include flow cytometersystems employing the optical configurations described above fordetecting light emitted by a sample in a flow stream. In certainembodiments, the subject systems are flow cytometer systems whichinclude one or more components of the flow cytometers described in U.S.Pat. Nos. 3,960,449; 4,347,935; 4,667,830; 4,704,891; 4,770,992;5,030,002; 5,040,890; 5,047,321; 5,245,318; 5,317,162; 5,464,581;5,483,469; 5,602,039; 5,620,842; 5,627,040; 5,643,796; 5,700,692;6,372,506; 6,809,804; 6,813,017; 6,821,740; 7,129,505; 7,201,875;7,544,326; 8,140,300; 8,233,146; 8,753,573; 8,975,595; 9,092,034;9,095,494 and 9,097,640; the disclosures of which are hereinincorporated by reference.

As described above, in some embodiments the subject systems areconfigured for imaging particles (e.g., cells) in sample flowing a flowstream, such as in the flow stream of a flow cytometer. The flow rate ofparticles in the flow stream may be 0.00001 m/s or more, such as 0.00005m/s or more, such as 0.0001 m/s or more, such as 0.0005 m/s or more,such as 0.001 m/s or more, such as 0.005 m/s or more, such as 0.01 m/sor more, such as 0.05 m/s or more, such as 0.1 m/s or more, such as 0.5m/s or more, such as 1 m/s or more, such as 2 m/s or more, such as 3 m/sor more, such as 4 m/s or more, such as 5 m/s or more, such as 6 m/s ormore, such as 7 m/s or more, such as 8 m/s or more, such as 9 m/s ormore, such as 10 m/s or more, such as 15 m/s or more and including 25m/s or more. For example, depending on the size of the flow stream(e.g., the flow nozzle orifice), the flow stream may have a flow rate inthe subject systems of 0.001 μL/min or more, such as 0.005 μL/min ormore, such as 0.01 μL/min or more, such 0.05 μL/min or more, such as 0.1μL/min or more, such as 0.5 μL/min or more, such as 1 μL/min or more,such as 5 μL/min or more, such as 10 μL/min or more, such as 25 μL/minor more, such as 50 μL/min or more, such as 100 μL/min or more, such as250 μL/min or more and including 500 μL/min or more.

The following examples are provided solely for further elucidation ofvarious aspects of the present teachings and are not intended toillustrate necessarily the optimal ways of implementing the teachings ofthe invention or the optimal results that can be obtained.

Example 1

A system similar to that described above in connection with FIG. 1 witha detection system similar to that described above in connection withFIG. 9A was employed to measure fluorescence radiation from polystyrenebeads stained with 8 discrete levels of fluorescence dyes, which aremarketed by Spherotech Inc. of Lake Forest, Ill. under tradenameRCP-30-5A. The system was also used to generate brightfield anddarkfield images in a manner discussed above.

FIG. 16A is a scatter plot of darkfield intensity versus brightfieldintensity. The rectangular section of the plot was used as a gate togenerate the data depicted in FIGS. 16B, 16C, and 16D, which containsabout 32% of all events measured (50,000 total events were detected).FIG. 16B shows a scatter plot of the red fluorescence (PI) v. greenfluorescence (FITC) emitted by each particle. This plot clearly shows 8populations with varying levels of brightness. FIGS. 16C and 16D arehistograms of the same data.

Example 2

FIRE, brightfield, and darkfield images of fixed peripheral bloodleukocytes stained with CD45-FITC and propidium iodide were obtainedusing a system similar to that described above in connection with FIG. 1with a detection system similar to that discussed above in connectionwith FIG. 9A. The sample also contained a fraction of live HeLa cells,stained with Calcein-AM. The cells were flowing through the flow cell ata rate of 0.5 meters/second during data acquisition.

The images shown in FIG. 17A are from top to bottom: overlay ofbright-field, CD45-FITC, and propidium iodide fluorescence channels,bright-field, dark-field, CD45-FITC, and PI channel fluorescence. Nocompensation has been applied, and all images are auto-scaled forviewing. Cells numbered 2, 3, 5, 8, 9, 12, 17, 20, and 21 are HeLa cells(B population), and the others are leukocytes (A population).

FIG. 17B is a scatter plot in which population A represents leukocytesand population B represents HeLa cells.

Example 3

FIRE, brightfield, and darkfield images of fixed peripheral bloodleukocytes stained with CD45-FITC and propidium iodide were obtainedusing a system similar to that described above in connection with FIG. 1with a detection system similar to that discussed above in connectionwith FIG. 9A. The sample was spiked with a small fraction of fixed MCF-7cells, stained with anti-EpCAM-FITC and propidium iodide (PI). The cellswere flowing through a flow cell at a rate of 0.5 meters/second duringdata acquisition.

The images shown in FIG. 18A are from top to bottom: an overlay ofbrightfield, FITC, and PI channel fluorescence, brightfield, darkfield,FITC, and PI fluorescence. In the leukocyte population, greenfluorescence is an artifact of fluorescence spillover from the PI stain.All images were auto-scaled in brightness for viewing, and thus,leukocytes appear to exhibit FITC fluorescence, yet this is a smallfluorescence spillover signal from the PI. Cells numbered as 1, 2, 4, 5,10, 13, 15, and 16 are MCF-7 cells.

FIG. 18B is a scatter plot in which population A represents leukocytes,and population B represents MCF-7 cells.

Those having ordinary skill in the art will appreciate that variouschanges can be made without departing from the scope of the presentteachings. In particular, various features, structures, orcharacteristics of embodiments discussed above can be combined in asuitable manner. For example, the detection systems discussed inconnection with one embodiment may be used in another embodiment.

The invention claimed is:
 1. A method comprising: applying aradiofrequency drive signal to an acousto-optic device; irradiating theacousto-optic device with a laser to generate a first angularlydeflected laser beam and a second angularly deflected laser beam;propagating the first angularly deflected laser beam along a differentoptical path from the second angularly deflected laser beam; andcombining the first angularly deflected laser beam with the secondangularly deflected laser beam and directing the combined laser beamonto a sample; and detecting one or more light signals from the sample.2. The method according to claim 1, wherein acousto-optic device is anacousto-optic deflector.
 3. The method according to claim 1, wherein thefirst angularly deflected laser beam comprises a local oscillator (LO)beam and the second angularly deflected laser beam comprises a pluralityof radiofrequency comb beams.
 4. The method according to claim 3,wherein the method comprises propagating the local oscillator beamthrough a top-hat beam shaper to produce a local oscillator beam with asubstantially constant intensity profile along a horizontal axis.
 5. Themethod according to claim 4, wherein the local oscillator beam has aGaussian intensity profile along a vertical axis.
 6. The methodaccording to claim 4, wherein the plurality of radiofrequency comb beamseach have a Gaussian intensity profile along the horizontal axis.
 7. Themethod according to claim 1, wherein each of the plurality ofradiofrequency comb beams irradiates a different spatial location on thesample.
 8. The method according to claim 7, wherein each of theplurality of radiofrequency comb beams overlaps with the localoscillator beam on the sample.
 9. The method according to claim 8,wherein fluorescence from the sample at each overlap location exhibits abeat frequency that comprises a frequency difference between the localoscillator beam and the radiofrequency comb beam.
 10. The methodaccording to claim 8, further comprising modulating the intensity ofeach of the radiofrequency comb beams.
 11. The method according to claim10, wherein modulating the intensity of the radiofrequency comb beamscomprises adjusting the intensity of each radiofrequency comb beam to beequal to the intensity of the local oscillator beam.
 12. The methodaccording to claim 11, wherein the detected light signal comprises atime-domain fluorescence emission signal.
 13. The method according toclaim 12, further comprising generating an image of the sample based ona plurality of time-domain fluorescence emission signals.
 14. The methodaccording to claim 13, wherein generating the image comprises frequencyde-multiplexing each fluorescence emission signal to determine the beatfrequencies at each overlap location on the sample.
 15. The methodaccording to claim 14, wherein frequency de-multiplexing comprisesobtaining a Fourier Transform of the fluorescence signal to determinethe beat frequency at each overlap location on the sample.
 16. Themethod according to claim 15, wherein generating the image comprisescomputing the amplitude of each beat frequency to provide a pixel valuecorresponding to a location of the image.
 17. The method according toclaim 16, wherein generating the image comprises generating a darkfieldimage of the sample and generating a brightfield image of the sample.18. The method according to claim 1, wherein the sample is present in aflow stream.
 19. The method according to claim 18, wherein the flowstream has a flow rate of particles of 1 m/s or more.